Polysaccharides which cross-reacted immunologically with Type III pneumococcal polysaccharide were isolated from two fungi, Pleurotis fornucopiae and Auricularia polytricha. The A. polytricha polysaccharide was found to contain three different biuronic acids, one of which was chromatographically similar to cellobiuronic acid, the basic structural unit of Type III pneumococcal polysaccharide. It also contained glucose, galactose, xylose, and large amounts of mannose. Chromatographically pure cellobiuronic acid was prepared from Type III pneumococcal polysaccharide by acid hydrolysis, followed by ion change and gel chromatography. Plant polysaccharides which had been subjected to mild acid hydrolysis and chromatography with DEAE sephadex were found to be non-immunogenic in hemolysis-in-gel tests, although the acid hydrolysis and DEAE chromatography had increased their reactivity with Type III pneumococcal antiserum in Ouchterlony testing.